• Journal of Welding and Joining
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• v.27 no.6
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• pp.31-35
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• 2009

When conducting CTOD test, especially in thick welded steel plate, fatigue pre-cracking occasionally failed to satisfy the requirements of standards thus making the test result invalid. Internally accumulated residual stress of test piece has been thought as one of the main reasons. The propagation of fatigue crack, started from the tip of machined notch, which might have propagated irregularly due to residual stress field. To overcome this kind of difficulty three methods to modify the residual stress are suggested in standard i.e. local compression, reverse bending and stepwise high-R ratio method. In this paper not only multi pass welding but also local pre-compressing process of thick steel plate has been simulated using finite element method for clarifying variation of internal welding residual stress. The simulated results show that welding residual stress is compressive in the middle section of the model and it is predominantly increased after machining the specimen. Comparing as-welded state all component of the welding residual stress changing to compressive in the tip of machine notch whereas residual stress of the outer area remain as tensile condition relatively. Analysis results also show that this irregular residual stress distribution is improved to be more uniformly by applying local compression.

• Journal of Korean Society of Steel Construction
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• v.13 no.4
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• pp.327-335
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• 2001

The characteristics and production mechanism of residual stress generated by multi-pass welding of the steel pipe were elucidated from the results of three-dimensional thermal elastic-plastic FEM analysis. When the steel pipe was jointed by multi-pass welding, the stress components of circumferential direction and radial direction near welded joints on the inner surface and the outer surface of the pope were tensile. The stress component of axial direction on the inner surface was tensile and on the outer surface was compressive. On the other hands, the production mechanism of residual stress generated by multi-pass welding of the steel pipe was investigated. Residual stress generated by welding of the steel pipe was investigated not only by the thermal history but also by geometrical shape. Then, the generality of the production mechanism of residual stress generated by multi-pass welding was confirmed.

• Proceedings of the KWS Conference
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• 2007.11a
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• pp.260-262
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• 2007

In nuclear power plants, residual stress of dissimilar metal weld propagates cracks in the weld metal which is susceptible to stress corrosion cracking. Overlay welding is a process widely used to mitigate residual stress replacing inside tensile stress by compressing stress. The purpose of this paper is to predict the effect of weld overlay by finite element analysis.

• Proceedings of the Korean Society of Laser Processing Conference
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• 2006.06a
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• pp.3-8
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• 2006

The integrity of laser welded structures is decided with fracture strength and fatigue strength. This study presents fracture behavior considering residual stress in the laser welding. Experiments are conducted and analyses are performed to explore the influence of residual stress on fracture behavior of bead-on laser welded compact specimen. Fracture experiments are performed using ASTM 1820. The performed analyses included thermo-elasto-plastic analyses for residual stress and subsequent J-integral calculation. A modified J integral is calculated in the presence of residual stresses. The J-integral is path-independent for combination of residual stress field and stress due to mechanical loading. The results indicates that the tensile residual stress near crack front bring the low fracture load while the compressive residual stress bring the high fracture load compared to no residual stress specimen. These results quantitatively understand the influence of residual stress on fracture behavior.

• International Journal of Naval Architecture and Ocean Engineering
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• v.10 no.2
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• pp.129-140
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• 2018

A residual stress generated in the steel structure is broadly categorized into initial residual stress during manufacturing steel material, welding residual stress caused by welding, and heat treatment residual stress by heat treatment. Initial residual stresses induced during the manufacturing process is combined with welding residual stress or heat treatment residual stress, and remained as a final residual stress. Because such final residual stress affects the safety and strength of the structure, it is of utmost importance to measure or predict the magnitude of residual stress, and to apply this point on the design of the structure. In this study, the initial residual stress of steel structures having thicknesses of 25 mm and 70 mm during manufacturing was measured in order to investigate initial residual stress (hereinafter, referred to as initial stress). In addition, thermal elastic plastic FEM analysis was performed with this initial condition, and the effect of initial stress on the welding residual stress was investigated. Further, the reliability of the FE analysis result, considering the initial stress and welding residual stress for the steel structures having two thicknesses, was validated by comparing it with the measured results. In the vicinity of the weld joint, the initial stress is released and finally controlled by the weld residual stress. On the other hand, the farther away from the weld joint, the greater the influence of the initial stress. The range in which the initial stress affects the weld residual stress was not changed by the initial stress. However, in the region where the initial stress occurs in the compressive stress, the magnitude of the weld residual compressive stress varies with the compression or tension of the initial stress. The effect of initial stress on the maximum compression residual stress was far larger when initial stress was considered in case of a thickness of 25 mm with a value of 180 MPa, while in case of thickness at 70 mm, it was 200 MPa. The increase in compressive residual stress is almost the same as the initial stress. However, if initial stress was tensile, there was no significant change in the maximum compression residual stress.

• Journal of Welding and Joining
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• v.24 no.2
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• pp.48-56
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• 2006

The fatigue strength of the welds is affected by such factors as the weld geometry, microstructures, tensile properties and residual stresses caused by fabrication. It is very important to evaluate the structural integrity of the welds in nuclear power plant because the weldment undergoes the most of damage and failure mechanisms. In this study, the fatigue assessments for a reactor vessel outlet nozzle with the weldment to the piping system are performed considering the welding residual stresses as well as the effect of local brittle zone in the vicinity of the weld fusion line. The analytical approaches employed are the microstructure and mechanical properties prediction by semi-analytical method, the thermal and stress analysis including the welding residual stress analysis by finite element method, the fatigue life assessment by following the ASME Code rules. The calculated results of cumulative usage factors(CUF) are compared for cases of the elastic and elasto-plastic analysis, and with or without residual stress and local brittle zone effects, respectively. Finally, the fatigue life of reactor vessel outlet nozzle weld is slightly affected by the local brittle zone and welding residual stresses.

• Journal of Welding and Joining
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• v.11 no.2
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• pp.53-61
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• 1993

One of the most interesting and promising steel groups considered for the rocket motor case, aircraft and aerospace component is the maraging(martensitic plus aging) nickel steel, developed by International Nickel Company in 1960. This material attains a very high strength with good fracture toughness by simple heat treatments which do not involve a quenching. Full strength can be obtained by "maraging" at 480.deg. for 3 hours for the 18% Ni maraging steel. The effect of heat treatments was considered on the residual stress field of 18% Ni maraging steel weldments. In experiments, various heat treatments such as stress relieve heat treatment, aging and solution heat treatment were carried out of the GTA weldments and the residual stresses were measured by using the hole drilling method. Whereas the conventional pattern of residual stress shows the stresses to be maximum along the weld centerline with tensile stress extending into the heat affected zone, the pattern in maraging steels shows the centerline stress to be compressive. After welding, a series of aging, solution heat treatment and solution heat treatment plus aging treatment were carried out and the residual stresses were measured to reveal that these heat treatments almost completely remove the welding residual stresses.

• International Journal of Korean Welding Society
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• v.1 no.1
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• pp.39-43
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• 2001

In this paper, the neutron diffraction is applied to the residual stress measurement on the 20mm-thick welded stainless steel plate ($100^{\circ}$$\times$100$mm^2$). The High Resolution Powder Diffractormeter of the Korea Atomic Research Institute The power of nuclear reactor was 24 MWt and the measured reflection in the 220 plane (2$\theta$ is $95^{\circ}$and wavelength is 1.8340 ${\AA}$ . The Poisson ratio of 0.265 and elastic constant of 211 GPa are applied to the calculation of stresses and strains. Three directional components such as normal, transverse, and longitudinal stresses are measured and the results show that the most of longitudinal stress is tensile and decreases, changing to compressive depending on the distance away from the welded spot. However, transverse component is changing from tensile to compressive along the depth of the welded point.

• Transactions of the Korean Society of Automotive Engineers
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• v.18 no.6
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• pp.38-45
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• 2010

Welding structures are designed to endure its expected life. The most important factors are life. Especially on welded structure, fatigue strength is critical. So this study performed a research on Box and T shape weldment specimen to examine the influence of welding type. In this experiment, the results indicate Box shape was available in more than T shape. Fatigue tests were performed to evaluate the fatigue strength of the both as-welded and statically pre-loaded specimens by 3 point bending load. Fatigue life can be improved by using Ultrasonic Impact Treatment(UIT) effect. Ultrasonic Impact Treatment(UIT) is excellent for eliminating the tensile residual stresses and generating compressive residual stresses which elevate fatigue strength of welded structures. Also, this shows that welding part has better fatigue life and welding was performed well. In this study, to evaluate the Ultrasonic Impact Treatment(UIT) effect, for welding structure, the experiment was conducted at various levels of stress range between 100MPa and 500MPa. From the test results, it was indicated that fatigue performance was improving by Ultrasonic Impact Treatment(UIT)

• Proceedings of the KSR Conference
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• 2004.06a
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• pp.872-877
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• 2004

Welding residual stresses are the main topics of welding research fields. The residual stresses and distortion of structures by welding exert negative effect on the safety of mechanical structures. That is, expansion of material by high temperature and distortion by cooling during welding process are caused by tensile and compressive residual stresses in welding material, and this residual stresses can induce fracture and fatigue problems of welding structures. The accurate prediction of residual stress and relaxation due to mechanical loading of weld zone is very important to improve the quality of weldment. In this study, a finite element modeling technique is developed to simulate the relaxation of residual stresses due to mechanical loading. The effects of load ratio for static and cyclic loading are evaluated based on analytical results.